Problem: There are 9 people riding on a train. At the previous stop, 20 people got off. How many people were riding the train before the stop?
Explanation: The number riding the train before the stop is the sum of the number who are riding now and the number who got off at the previous stop. The sum is $9 + 20$ people on the train. $9 + 20 = 29$ people were on the train.